Electrically Pumped 1.5-µm MEMS VCSEL Tunes over 40 nm

Breck Hitz

A collaboration of scientists at Two-Chip Photonics AG and Technische Universität Darmstadt, both in Darmstadt, Germany, and at Technische Universität München in Munich, Germany, has resulted in an electrically pumped vertical-cavity surface-emitting laser (VCSEL) with a 40-nm tuning range, the widest yet reported for such a device. Lasing in a single mode from ~1553 to ~1595 nm, the laser could find important applications not only in telecommunications, but also in trace-gas sensing, metrology, process control and medical diagnostics.

Figure 1. Passing a current (injected through the vias) through the mirror membrane expands it and thereby increases the resonator length, tuning the wavelength of the electrically pumped VCSEL. The chip consisting of the mirror membrane and its GaAs substrate is grown separately and placed, upside down, above the gain chip. ©2004 IEEE.

Mechanically, the laser consists of two chips: a VCSEL amplifier and a microelectromechanical systems (MEMS) mirror membrane (Figure 1). The inherent curvature of the mirror membrane -- its radius of curvature is roughly 5.5 mm -- removes the need for critical alignment between the two chips when they are assembled. This mirror membrane, which serves as the output coupler, and a gold mirror/dielectric distributed Bragg reflector combination at the bottom of the semiconductor define the resonator.

The laser is tuned by running a current through the mirror membrane, heating it and causing it to expand. As it expands, it lengthens the laser resonator, shifting the output to a longer wavelength. (In the figure, it appears that the mirror membrane makes electrical contact with the semiconductor's N-contact. In fact, the bottom layers of the membrane are undoped and, hence, nonconductive, so the membrane is insulated from the semiconductor current.)

Figure 2. The lower trace indicates the spectral output of the laser tuned to its maximum output power. The envelope (upper trace) indicates the maximum power obtained across the more-than-40-nm tuning range. ©2004 IEEE.

Output power from the laser is relatively constant as it is tuned across its ~40-nm tuning range (Figure 2). The maximum power output is around 100 µW; two readily addressable factors account for this low value. The scientists estimate that the reflectivity of the mirror membrane is as high as 99.95 percent, which is far from the desired optimum coupling even for this low-gain laser. Also, the presence of higher-order side modes in Figure 2 indicates that the resonator's geometric parameters (mirror curvature, cavity length and intracavity aperture formed by the buried-tunnel junction) are not optimized for single-mode oscillation. They believe that improvements in both areas will result in considerably greater output power.

Because tuning speed is a critical parameter in many applications, they undertook a measurement of their thermoelectrically tuned laser's wavelength-tuning frequency response. They found that the 3-dB cutoff frequency was approximately 500 Hz and that the 1/e-time constant for a step change was about 1 ms. Although these specifications are slower than the corresponding ones for an electrostatically tuned MEMS laser, they are adequate for many applications, the scientists say.